This invention relates generally to thermal cycling switches of the type used for controlling resistance-type power loads, such as surface heating units of an electric cooking range, and more particularly to infinitely variable thermal cycling switches of the voltage-sensitive type.
Thermal cycling switches, also known as "infinite switches," use a rotating cam controlled by an external control knob to provide the actuation of switch elements inside an insulated housing, which is preferably made as small as possible consistent with safety requirements for easy mounting on control panels for range units. The rotating cam operates an ON/OFF switch for one of the lines, as well as the cycling switch, to provide complete electrical isolation of the power load when the switch is in the OFF position, and the cam may also operate contacts for a pilot light to show that power is connected to the load whenever the switch is in the ON position.
In one type of switch, the cam includes a variable cam surface which is adapted to move an adjusted contact on a contact arm to variable positions with respect to a cycling contact mounted on a cycling arm. The cycling arm includes a bimetal member that is heated whenever the cycling arm contact engages the adjusted contact to energize the power load. As the bimetal heats up, it tends to deflect the cycling arm away from the adjusted contact to break the circuit and stop the heating of the bimetal as well as the load. As the bimetal then cools, it moves the cycling contact back toward the adjusted contact to reconnect the circuit and reheat the bimetal and the load. The further the adjusted contact is positioned by the cam surface away from the normal position of the cycling contact under ambient temperature conditions, the longer the OFF or open circuit portion of the complete cycle. Generally, the cam surface is formed to provide an OFF position at which the adjusted contact is held permanently away from the cycling contact, and the cam surface progressively moves the adjusted contact all the way to a high or full ON position, at which the contacts remain in engagement at all times.
In an alternative type of thermal cycling switch, the contact corresponding to the adjusted contact may be in a fixed position, and the cam used to provide a variable bias to the cycling arm, which includes the bimetal member, to vary the force applied by the bimetal, and hence achieve a variable cycling relation for the cycling arm.
One problem with the latter type of switch with the fixed contact is that the gradual movement of the cycling arm under the heating and cooling of the bimetal tends to result in creep action between the contacts, which increases the likelihood of arcing and possible welding of the contacts. In switches of this type, it has been found necessary to use some form of snap action, either in the form of an overcenter spring or a magnet adjacent the stationary contact, to provide a snap action for opening and closing to eliminate the problems associated with contact creep.
On the other hand, in the case of switches of the former type, in which the adjusted contact is movable by a cam responsive to a control knob, it has been found possible to provide a snap action for the cycling bimetal arm by an arrangement first disclosed in U.S. Pat. No. 2,623,137, issued Dec. 23, 1952 to W. H. Vogelsberg. In this case, the current does not run through the bimetal itself but, rather, through a heater which is insulated from the bimetal and spaced therefrom along most of its length by the insulation. The bimetal is arranged so that the high expansion side carries the cycling contact which engages the adjusted contact, while the lower expansion side is adjacent the heater. When the contacts are closed, the full current to the power load goes through the heater strip, which is connected at the end to the bimetal adjacent the contact, so that the heater strip heats up more rapidly than the bimetal, which is not only electrically, but also thermally, insulated from the heater by the insulating strip. Since the heater is connected to the end of the bimetal and expands more rapidly than the bimetal, the initial heating action caused by expansion of the heater itself tends to deflect the bimetal in a direction against the high expansion side and move the contacts more closely together. Only after the heat has been applied for a sufficient time is the bimetal itself heated enough to overcome the action of the thermal expansion of the heater and cause the bimetal to deflect to move the cycling contact away from the adjusted contact to open the circuit and discontinue heating. As the heater and bimetal both cool, the cycling contact moves back toward engagement with the adjusted contact. A more commercial version of this type of switch is shown in U.S. Pat. No. 3,634,802, granted Jan. 11, 1972 to George C. Aldous, assigned to the assignee of the present invention and incorporated by reference herein. This patent shows additional ways of adjusting the action of the cycling contacts to provide the desired length of cycle and to provide compensation for changing ambient temperatures which would otherwise affect the cyclic operation of the bimetal member.
In the above type of switch, the current through the heater strip represents the full current passing through the load and, as a result, the characteristics of the switch must be adjusted to a given load in terms of current flow, since a higher or lower current flow required by a higher or lower wattage load will change the cyclic operation so that the settings of the switch at a given angular position are only repeatable for a given amperage of current, and hence given wattage of the power load that is being controlled.
The foregoing characteristics of such switches require that switches be matched to loads and, while this is not a matter of great difficulty on an assembly line, it does make a problem for replacement purposes since, in the case of surface units of electric ranges, different types of ranges may have surface units of substantially different wattage, depending upon the diameter of the surface unit, as well as whether or not it is a high or low powered unit for a given size. This requires that service personnel be supplied with a number of different switches for replacements in the field, which tends to result in a substantially increased inventory requirement. For this reason, it has been recognized as desirable to have a thermal cycling switch which is relatively insensitive to the wattage of the load. This can be accomplished as shown, for example, in the patent of H. F. Hild et al. U.S. Pat. No. 3,110,789, granted Nov. 12, 1963, by utilizing a heater arrangement which is in parallel rather than in series with the load, and therefore responsive only to the applied voltage rather than the current conducted by the switch. With this arrangement, no strip heater is used, but rather the bimetal member is sandwiched between an insulator, and then a low wattage, high resistance heater wire is wrapped around the insulated bimetal over a substantial length so that as the wire is heated, the resultant heat will be transmitted through the insulator to the bimetal member, which will then deflect to move the cycling contact with a snap action provided by the magnet at the stationary contact. The entire bimetal arm is biased by means of a second angularly extending arm, which may be formed of a compensating bimetal to correct for ambient conditions, and the force applied through still another cam follower arm from the rotary cam to provide a variable bias to the cycling arm to control the cycling action. However, with this arrangement, the component parts become more complex, as does the adjustment for the compensating bimetal, and the snap action depends upon a relatively small permanent magnet to retard the opening of the contacts during the cycling action.